Because of the increasing emphasis on producing lower weight automobiles in order, among other things, to conserve energy, considerable effort has been directed toward developing aluminum alloy products suited to automotive application. It is appreciated that different components on the automobile can require different properties in the form used. For example, an aluminum alloy sheet when formed into shaped outside body panels should be capable of attaining high strength which provides resistance to denting as well as being free of Lueders' lines, whereas the strength and the presence or absence of such lines on inside support panels, normally not visible, is less important. Lueders' lines are lines or markings appearing on the otherwise smooth surface of metal strained beyond its elastic limit, usually as a result of a multi-directional forming operation, and reflective of metal movement during that operation. Bumper applications on the other hand require such properties as high strength, plus resistance to denting and to stress corrosion cracking and exfoliation corrosion, usually together with receptiveness to chrome plating. To serve in inside body panel automotive applications, an aluminum alloy product needs to possess good forming characteristics to facilitate shaping, drawing, bending and the like, without cracking, tearing, or excessive wrinkling or press loads, and yet be possessed of adequate strength and good corrosion resistance. Since forming is typically carried out at room temperature, formability at room or low temperatures is often a principal concern. Still another aspect which is considered important in automotive uses is weldability, especially resistance spot weldability. For example, the outside body sheet and inside support sheet of a dual sheet structure such as a hood, door or trunk lid are often joined by spot welding, and it is important that the life of the spot welding electrode is not unduly shortened by reason of the aluminum alloy sheet so as to cause unnecessary interruption of assembly line production, as for electrode replacement. Also, it is desirable that such joining does not require extra steps to remove surface oxide, for example. In addition, the alloy should have high bending capability without cracking or exhibiting orange peel, since often the structural products are fastened or joined to each other by hemming or seaming.
Various aluminum alloys and sheet products thereof have been considered for automotive applications, including both heat treatable and non-heat treatable alloys. Heat treatable alloys offer an advantage in that they can be produced at a given lower strength level in the solution treated and quenched temper which can be later increased by artificial aging after the panel is shaped. This offers easier forming at a lower strength level which is thereafter increased for the end use. Further, the thermal treatment to effect artificial aging can sometimes be achieved during a paint bake treatment, so that a separate step for the strengthening treatment is not required. Non-heat treatable alloys, on the other hand, are typically strengthened by strain hardening, as by cold rolling. These strain or work hardening effects are usually diminished during thermal exposures such as paint bake or cure cycles, which can partially anneal or relax the strain hardening effects.
Accordingly, it would be advantageous to provide robust sheet materials having an excellent combination of formability and corrosion resistance as well as good strength.
The primary object of the present invention is to provide a method for forming an aluminum sheet product and having a combination of excellent formability and corrosion resistance as well as good strength.
Another objective of the present invention is to provide a composition that it capable of being formed into an aluminum sheet product which has considerably improved characteristics, particularly in formability and corrosion resistance.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.